1. Field of the Invention
The present invention relates to a method of signal processing in a hearing aid system. The invention, more specifically, relates to a method of binaural noise suppression in a hearing aid system. The invention further relates to hearing aids and hearing aid systems having means for noise suppression.
Within the present disclosure a hearing aid system is generally understood as meaning any system which provides an output signal that can be perceived as an acoustic signal by a user, or contributes to providing such an output signal, and which has means which are used to compensate an individual hearing loss of the user or contribute to compensating the hearing loss of the user or contribute to compensating the hearing loss. These systems may comprise hearing aids which can be worn on the body or on the head, in particular on or in the ear, and hearing aids which can be fully or partially implanted. However, devices whose main aim is not to compensate for a hearing loss, for example consumer electronic devices (televisions, hi-fi systems, mobile phones, MP3 players etc.), may also be considered hearing aid systems, provided they have measures for compensating for an individual hearing loss.
Within the present context a hearing aid can be understood as a small, battery-powered, microelectronic device designed to be worn behind or in the human ear by a hearing-impaired user. Prior to use, the hearing aid is adjusted by a hearing aid fitter according to a prescription. The prescription is based on a hearing test, resulting in a so-called audiogram, of the performance of the hearing-impaired user's unaided hearing. The prescription is developed to reach a setting where the hearing aid will alleviate a hearing loss by amplifying sound at frequencies in those parts of the audible frequency range where the user suffers a hearing deficit. A hearing aid comprises one or more microphones, a battery, a microelectronic circuit comprising a signal processor, and an acoustic output transducer. The signal processor is preferably a digital signal processor. The hearing aid is enclosed in a casing suitable for fitting behind or in a human ear.
Within the present context a hearing aid system may comprise a single hearing aid (a so called monaural hearing aid system) or comprise two hearing aids, one for each ear of the hearing aid user (a so-called binaural hearing aid system). Furthermore the hearing aid system may comprise an external device, such as e.g. a smart phone having software applications adapted to interact with other devices of the hearing aid system. Thus within the present context the term “hearing aid system device” may denote a hearing aid or an external device.
In an open space, sound waves propagate generally in straight lines, i.e. directly from point to point. A hard surface may reflect a sound wave. The reflected wave is referred to as an echo. In a space with a hard surface sound propagation from point-to-point may be a combination of a direct wave and an echo. The echo will be delayed due to the longer path, comparing to the direct wave. In a space with multiple hard faces propagation from point-to-point may be by a direct wave and by a multitude of echoes, some of which having bounced many times.
Reverberation is the persistence of sound in a particular space after an original sound has been provided. A reverberation is created when a sound is provided in an enclosed space causing a large number of echoes to build up and then slowly decay as the acoustic energy is absorbed by the walls and air. This is most noticeable when the sound source stops while the reflections continue, decreasing in amplitude, until they can no longer be heard. Reverberation is the aggregate of many thousands of echoes that arrive in very quick succession (0.01-1 milliseconds between echoes). As time passes, the volume of the aggregated echoes decays until the echoes cannot be heard at all.
Often the first say 100 milliseconds of the reverberation is denoted the early reflections, and the remaining part is denoted the late reverberation. It is well known that the early reflections generally may enhance speech intelligibility, while the late reverberation generally is detrimental.
Reverberation is known to have a detrimental effect on speech intelligibility, spatial separation, localization, cognitive load, listening effort and listening comfort. Although moderate amounts of reverberation do not affect speech recognition performance by normal-hearing listeners, it has a detrimental effect on speech intelligibility by hearing impaired and elderly listeners.
Reverberation is particularly a problem in untreated rooms with hard surfaces, where the reflections from the walls interfere with the direct sound, causing both reduced listening comfort and lower speech intelligibility. A few examples of demanding acoustic environments include large public spaces such as indoor train stations, shopping malls and canteens but also smaller rooms such as modern open kitchens. The problem is worsened when there are multiple acoustic sources present, that degrade the target-to-interferer noise ratio.
The detrimental effects of reverberation may, on a general level, be divided into two categories namely overlap-masking and self-masking. Overlap-masking is caused by the overlap of reverberant energy of a preceding phoneme on the following phoneme. This effect is particularly evident for low-energy consonants preceded by high-energy voiced segments (e.g., vowels). The additive reverberant energy fills in the gaps and silent intervals (e.g., stop closures) associated with vocal tract closures. An example of this effect is the words “cab” and “cat” where the high energy vowel masks the low energy consonant which causes consonant confusion which leads to a decrease in intelligibility. Self-masking is caused by the internal smearing of energy within each phoneme. This effect is particularly evident in reverberant sonorant sounds (e.g., vowels), where the format transitions become flattened. Generally, the self-masking effect is substantially smaller compared to the overlap-masking of consonants.
It is well known that people with normal hearing can usually follow a conversation despite being in a situation with several interfering speakers and significant background noise. This situation is known as a cocktail party environment. As opposed hereto hearing impaired people will typically have difficulties following a conversation in such situations. The same is true with respect to hearing in reverberant rooms.
2. The Prior Art
A method for suppression of room reverberation, using the signals recorded by two spatially separated microphones, is disclosed in the article by Allen et al.: “Multimicrophone signal-processing technique to remove room reverberation from speech signals”, Journal Acoustical Society America, vol. 62, no. 4, pp. 912-915, October 1977. According to this method the individual microphone signals are transformed into short-term spectra and divided into frequency bands whose corresponding outputs are co-phased (delay differences are compensated), and the gain of each frequency band is set based on the cross correlation of the short-term spectra of the individual microphone signals.
WO-A1-2012007183 discloses a method of processing signals in a hearing aid system comprising the steps of transforming two audio signals to the time-frequency domain, calculating a value representing the interaural coherence, deriving a first gain based on the interaural coherence, applying the first gain value in the amplification of the time-frequency signals, and transforming the signals back into the time domain for further processing in the hearing aid in order to alleviate a hearing deficit of the user of the hearing aid system, and wherein the relation determining the first gain value as a function of the value representing the interaural coherence comprises three contiguous ranges for the values representing the interaural coherence, where the maximum slope in the first and third range are smaller than the maximum slope in the second range and wherein the ranges are defined such that the first range comprises values representing low interaural coherence values, the third range comprises values representing high interaural coherence values and the second range comprises values representing intervening interaural coherence values.
WO-A1-2011006496 discloses a hearing aid system having a processing unit that comprises a first microphone and a second microphone, wherein the output of the first microphone is operationally connected to a first input of a subtraction node and the output of the second microphone is operationally connected to the input of an adaptive filter. The output of the adaptive filter is branched and in a first branch operationally connected to the second input of the subtraction node and in a second branch operationally connected to the input of the remaining signal processing in a hearing aid. The output from the subtraction node is operationally connected to a control input of the adaptive filter.
US-A1-20080212811 discloses a signal processing system with a first signal channel having a first filter and a second signal channel having a second filter for processing first and second channel inputs and producing first and second channel outputs, respectively. Filter coefficients of at least one of the first and second filters are adjusted to minimize the difference between the first and second channel outputs. The resultant signal match processing of the signal processing system gives broader regions of signal suppression than using Wiener filters alone for frequency regions where the interaural correlation is low, and may be more effective in reducing the effects of interference on the desired speech signal. The filtering in the first and second signal channels are carried out in the frequency domain.
US-A1-20120328112 discloses a method for reduction of reverberation in binaural hearing systems. This has been done by developing a method for obtaining a reduced-reverberation, binaural output signal, for a binaural hearing apparatus. First of all, a left input signal and a right input signal are provided. The two input signals are combined to form a reference signal. The reference signal is used to ascertain spectral weights, or these weights are provided in another way, in order to use them to reduce late reverberation. To this end, the two input signals have the spectral weight applied to them. Furthermore, a coherence for signal components of the weighted input signals is ascertained. Non-coherent signal components of both weighted input signals are then attenuated in order to reduce early reverberation.
It is a general problem for the prior art that the methods for binaural suppression of reverberation and noise suffer from sound artifacts. This may impair speech intelligibility and listening comfort for a hearing aid user.
It is therefore an object of the present invention to provide an improved method of processing in a hearing aid that can relieve the detrimental effects of reverberation.
It is another object of the present invention to provide a hearing aid system comprising improved means adapted for relieving the detrimental effects of reverberation.
It is yet another object of the present invention to provide a method and a hearing aid system adapted for improving the listening comfort for a hearing aid user.
It is still another object of the present invention to provide a method and a hearing aid system adapted for improving the suppression of uncorrelated noise in a binaural hearing aid system.
Finally it is another object to provide improved suppression of correlated noise.